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I want to buy an air compressor and need to figure out how big of a tank and how big of a motor I need.
I don't do this for a living. I will use it for occasionally running an impact wrench or rotary tool. Occasionally I clean something with air. One regular job I would need it to be able to handle is this- I have a 1500 square foot work shop that gets very dusty, grows big dust bunnies along the ceiling beams. The easiest way to clean it is to cover everything in the shop and hit it with air with a wand.
How big of a compressor tank/motor, and any models that you folks would recommend is much appreciated.

For intermittent impact wrench use you don't really need much capacity. Even a small portable compressor/tank will handle that. One of the biggest limitations here though is these units usually have only a 1/4" outlet, and that causes a ton of pressure drop at higher flows.

Rotary cutoff tools are a little trickier, especially if you want to run it for more than a couple minutes at a time.

All a larger tank will do capacity wise is give you more minutes of run time before you need to take a break to let the compressor catch up (and it helps with keeping the compressor from having to cycle on and off too often). But tank capacity will never make up for having too small a compressor for anything run for long periods, like more than a minute or two continuously.

I've just got a 120V compressor, I think it can make about 3.5CFM at 90psig. If I let it run up to 130psig (I have about 30 gallons compressed air storage) I can run the cheapie HF cutoff tool at 80psig for maybe three or four minutes before my pressure drops below 80 psig. Pretty much a ditto for air blasting or sandblasting with a small pocket portable sandblaster.

So that's about all you can get on a 120V 20amp circuit. If you need more air than that you're looking most likely at one of the upright 240VAC stationary units.

If you use long hoses etc you might want to just get 1/2" hose right up front or at least 3/8" hose. 1/4" is too small in many cases, ditto on the quick connects. Yes most of these airtools do have only a 1/4" NPT connection at the tool though.

What I've seen on garagejournal is people like to pick up the good old known high quality units like Quincy and others off craigslist. Sometimes the motor is burned out but the compressor is often ok. That seems like a pretty good way to get a unit at a reasonable price. The new 'oilless' designs don't last as long as the good 'ol reciprocating type.

Rotary tools take a lot. I have a 4 foot high tank, can wrap my arms around it...however big that is. Anyway with the rotary tools it drains quickly and needs to cycle.

I would lean towards getting more than you need, you're likely to use it. But everyone is different, you may know that you'll never really need more capacity. I'm usually trying, expanding, etc so it was good that I listened to folks that said get the most I could for my first one - and it still fits well for my needs.

I am glad I didn't go any larger though - mine has wheels and is "barely" portable. I mean not that most people need one to be but on a few occasions it has been nice to be able to load mine by myself and take it somewhere. Nice to have that option if needed, it's at my office instead of my garage right now for instance...though maybe that's a bad thing, I could use it right now at home!

I'd have to agree with davebugs about staying away from oil less. Unless it's just something you use occasionally, or don't mind a lot of noise. Whats your budget? If its in the $1000 + range there are a lot of great two stage 5hp compressors out there.

For my personal use i just have an Makita mac2400. It's relatively quiet,, small, and It puts out 4.2CFM at 90 psi. It keeps up with my nailers, but i do have to wait on it if i'm using impact guns sometimes. It would not keep up with blowing out an entire shop with an air wand though.

It would not keep up with blowing out an entire shop with an air wand though.

oh yeah i forgot i've used mine for that before, blowing off a sanded down deck, and it's like a 40 gallon or something and it needs a lot of break time for catching up.

mine is an oil-less variety. i didn't know the difference when i bought it, first time ever touching an air tool literally, none of my family or friends ever had one. it's 7 years old now and still going, guess i'm lucky so far and should think about what i'm getting next.

Mine's an oilless portable my dad bought from Sears probably 15 years ago. I think they can last a long time in intermittent use, but probably aren't really intended for daily use for years and years. They are loud though. Would probably make a great base for a vibration table.

I also found out using an SSR solid state relay to switch the oilless type does not work. I think I almost ruined the compressor trying it. It switched the compressor on OK but for some reason when the SSR was supposed to turn off it was like it interrupted only 1/2 the waveform or something and the compressor made some very strange sounds until I could get to it to unplug it. Then it wouldn't even start back up when I plugged it back in but the next day it seemed to be working fine again.

I also found out using an SSR solid state relay to switch the oilless type does not work. I think I almost ruined the compressor trying it. It switched the compressor on OK but for some reason when the SSR was supposed to turn off it was like it interrupted only 1/2 the waveform or something and the compressor made some very strange sounds until I could get to it to unplug it. Then it wouldn't even start back up when I plugged it back in but the next day it seemed to be working fine again.

Do you still know the brand and part number of the SSR?

It sounds like the SSR was overhead. Were you using a heat sink? If not, it might still be salvageable.

The SSR was inexpensive, maybe $10 at the most. I wasn't really worried about the SSR dying. If it had killed the compressor (Which thankfully it didn't) that would be far worse of course.

I thought I had read SSR's can switch motor loads, but, thinking about it, these 'oilless' compressors, I don't think they have a motor in them, I think it's some kind of almost transformer like vibrating rod with some kind of valves, just based on the way it BUZZZES when it runs rather than sounding like a motor.

This was the SSR: http://www.amazon.co...ils_o00_s00_i00Yes it did have a heat sink. It only ran for a couple minutes and when I tried to interrupt the power it went all wonky and made the compressor make some tortured sounds.

It's completely possible the inrush on this compressor is quite a bit more than 40 amps. Hm this SSR seems to indicate resistive loads only maybe just the wrong SSR for the application. Anyway I will use a good quality contactor instead if I try to do that in the future.

It's completely possible the inrush on this compressor is quite a bit more than 40 amps. Hm this SSR seems to indicate resistive loads only maybe just the wrong SSR for the application. Anyway I will use a good quality contactor instead if I try to do that in the future.

Huh, that relay should have been just fine, even with the inrush current. Sorry for going off on a tangent. I do a lot with motors and motor controls at work, so just get curious. I really enjoy controls and control logic.

Yah I thought that SSR would have been OK, I don't know, maybe it was defective. But I really think the 'oilless' types are some strange non-linear, not quite motor like load that maybe the SSR's don't like at all. I just hardwired switches to control the on/off and the air supply solenoid, since my black pipe air system has a few small leaks in it. So the compressor is in a separate building from the main garage so I don't have to listen to it run, but I have a few switches around to turn it on remotely basically.

The motors of an oil-less are no different in design than that of a similar oiled head unit.

Only catch is that you must make certain you know what type of current the motor is designed for. AC or DC. Compressors old and new come in both flavors, regardless of oiled or not. And when you get big enough the motors will utilize a capacitor for starting and sometimes running.

The DC motors are a fantastic find if you want to make a good generator. Ive had my eyes open for one for a while now.

Hm interesting, I'm trying to come up with a setup I can parallel my generator with the utility, I was thinking of using like a 3HP or 5HP induction motor and run it as an induction generator, I was working on that here, any info invited: http://www.garagejou...ad.php?t=174245